An ever-increasing number of relatively inexpensive, low power wireless data communication services, networks and devices have been made available over the past number of years, promising near wire speed transmission and reliability. Various wireless technology is described in detail in the 802 IEEE Standards, including for example, the IEEE Standard 802.11a (1999) and its updates and amendments, the IEEE Standard 802.11n (2009), and the IEEE draft standards 802.15.3, and 802.15.3c now in the process of being finalized, all of which are collectively incorporated herein fully by reference.
Some communication systems compliant with the IEEE 802.11b, for example, use only a single-carrier (SC) mode in which information is transmitted using only one carrier. Other transmission systems compliant with the IEEE 802.11a and 802.11g standards (or the “802.11a/g standard”) as well as the IEEE 802.11n standard achieve their high data transmission rates using Orthogonal Frequency Division Multiplexing (OFDM) encoded symbols mapped up to a 64 quadrature amplitude modulation (QAM) multi-carrier constellation. Generally speaking, the use of OFDM divides the overall system bandwidth into a number of frequency sub-bands or channels, with each frequency sub-band being associated with a respective sub-carrier upon which data is modulated. Thus, each frequency sub-band of the OFDM system can be viewed as an independent transmission channel within which to send data, thereby increasing the overall throughput or transmission rate of the communication system.
Generally regarding OFDM, transmitters used in the wireless communication systems that are compliant with the aforementioned IEEE 802.11a/g and 802.11n standards perform multi-carrier OFDM symbol encoding (which often includes error correction encoding and/or interleaving), convert the encoded symbols into the time domain using Inverse Fast Fourier Transform (IFFT) techniques, and perform digital to analog conversion and conventional radio frequency (RF) upconversion on the signals. These transmitters then transmit the modulated and upconverted signals after appropriate power amplification to one or more receivers. Likewise, receivers used in the wireless communication systems that are compliant with the aforementioned 802.11a/g and 802.11n standards generally include an RF receiving unit that performs RF downconversion and filtering of the received signals (which may be performed in one or more stages), and a baseband processor unit that processes the OFDM encoded symbols bearing the data of interest. Generally, the digital form of each OFDM symbol presented in the frequency domain is recovered after baseband downconversion, conventional analog to digital conversion and Fast Fourier Transformation (FFT) of the received time domain analog signal. Thereafter, the baseband processor performs frequency domain equalization (FEQ) and demodulation to recover the transmitted symbols, and these symbols are then processed in a Viterbi decoder to estimate or determine the most likely identity of the transmitted symbol. The recovered and recognized stream of symbols is then decoded, which often includes deinterleaving and/or error correction using any of a number of known error correction techniques, to produce a set of recovered signals corresponding to the original signals transmitted by the transmitter.
To improve transmission performance, it is known to use multiple transmit and receive antennas within a wireless transmission system. In general, multiple transmit and receive antennas increase the number of signals which may be propagated in the communication system and/or compensate for deleterious effects associated with the various propagation paths. Such systems are commonly referred to as a multiple-input, multiple-output (MIMO) wireless transmission systems. The term also may be used to refer to degenerate forms of MIMO such as single-input, multiple-output (when only the receiver includes multiple antennas) and multiple-input, single-output (when only the transmitter has multiple antennas). These systems are specifically provided for within the 802.11n IEEE Standard. Generally speaking, the use of MIMO technology produces significant increases in spectral efficiency and link reliability of multiple antenna communication systems, and these benefits generally increase as the number of transmission and receive antennas within the MIMO system increases.
Generally, a MIMO channel formed by the various transmit and receive antennas between a particular transmitter and a particular receiver includes a number of independent spatial channels. As is known, a wireless MIMO communication system can provide improved performance (e.g., increased transmission capacity) by utilizing the additional dimensionalities created by these spatial channels for the transmission of additional data. Of course, the spatial channels of a wideband MIMO system may experience different channel conditions (e.g., different fading and multi-path effects) across the overall system bandwidth and may therefore achieve different signal to noise ratios (SNRs) at different frequencies (i.e., at the different OFDM frequency sub-bands) of the overall system bandwidth. Consequently, the number of information bits per modulation symbol (i.e., the data rate) that may be transmitted using the different frequency sub-bands of each spatial channel for a particular level of performance may differ from frequency sub-band to frequency sub-band.
Thus, in MIMO wireless communication systems, RF modulated signals generated by a transmitter may reach a particular receiver via a number of different spatial channels and, in the case of OFDM, a number of different frequency channels, the characteristics of which typically change over time due to the phenomena of multi-path and fading. To compensate for the time varying, frequency selective nature of the propagation effects on the propagation channels (spatial channels and/or frequency channels), and generally to enhance effective encoding and modulation in a wireless communication system, each receiver of the wireless communication system may periodically develop or collect channel state information (CSI) for each of the propagation channels. Generally speaking, CSI is information defining or describing one or more characteristics about each of the channels (for example, the gain, the phase and the SNR of each channel). Upon determining the CSI for one or more channels, the receiver can construct a channel model to coherently combine signal components received via respective propagation channels.
However, instead of using the various different transmission and receive antennas to form separate spatial channels on which additional information is sent, transmission and reception properties can be improved in a MIMO system by using each of the various transmission antennas of the MIMO system to transmit the same signal while phasing (and amplifying) this signal as it is provided to the various transmission antennas to achieve beamforming or beamsteering. Generally speaking, beamforming or beamsteering creates a spatial gain pattern having one or more high gain lobes or beams (as compared to the gain obtained by an omni-directional antenna) in one or more particular directions, while reducing the gain over that obtained by an omni-directional antenna in other directions. If the gain pattern is configured to produce a high gain lobe in the direction of each of the receiver antennas, the MIMO system can obtain better transmission reliability between a particular transmitter and a particular receiver, over that obtained by single transmitter-antenna/receiver-antenna systems.
There are many known techniques for determining a steering matrix specifying the beamsteering coefficients that need to be used to properly condition the signals being applied to the various transmission antennas so as to produce the desired transmit gain pattern at the transmitter. As is known, these coefficients may specify the gain and phasing of the signals to be provided to the transmitter antennas to produce high gain lobes in particular or predetermined directions. These techniques include, for example, transmit-MRC (maximum ratio combining) and singular value decomposition (SVD).
In general, devices in systems compliant with 802.11a/g/n standards communicate using communication frames transmitted in data units that include a physical-layer (PHY) preamble followed by the communication frame, i.e., header and data portions. The PHY preamble is used for automatic gain control (AGC) setting, antenna diversity selection, timing acquisition, coarse frequency recovery, data unit and frame synchronization, and channel estimation. When receiving a data unit, a receiver typically derives channel information using the preamble, and accordingly applies coherent combining techniques only to the header and payload portions of the data unit. As a result, the receiver processes the beginning (i.e., the earlier-in-time portion) of the preamble with lower sensitivity and reliability.